U.S. Pat. Nos. 3,494,709 and 3,536,121 to Piearcey disclose the basic utility and method of making single crystal superalloy gas turbine blades. Since the early 1970's date of the patents, there has been considerable research and development applied to single crystal turbine blades, to the point where they are presently in use in both military gas turbine aircraft engines and in commercial aviation. The especially attractive feature of single crystal gas turbine blades is their high temperature creep strength. Since single crystal superalloys retain their strength to higher temperatures than polycrystalline counterparts, there is less need for cooling air and higher engine efficiencies result.
The single crystal approach has extended the upper temperature capability of nickel superalloys into the range 1000.degree.-1200.degree. C. The thrust of single crystal alloy and heat treatment development has been to further improve the high temperature utility of this class of material. See U.S. Pat. No. 4,209,348 to Duhl et al and U.S. Pat. No. 4,222,794 to Schweizer et al. Typically, single crystal turbine articles are hollow; they are cast to nominal size and machined at some locations. All the foregoing patents are of the present assignee and their disclosures are hereby incorporated by reference.
Nickel superalloys in general, including single crystal alloys, owe their excellent high temperature properties to the presence of the coherent ordered precipitate gamma prime. Since the days of the early superalloys, it has been an aim to increase the volume fraction of gamma prime. However, in achieving this objective, the resultant alloys have become less forgeable. For example, an early alloy such as Waspaloy was easily worked above the gamma prime solvus temperature. More recent intermediate strength alloys such as Astroloy are worked with difficulty below the gamma prime solvus: low strain rates must be used to avoid grain boundary cracking. And since the temperature of the billet is critical, warm die forging is generally used; intermediate annealing steps are prevalent. The highest strength superalloys with very high volume fractions of gamma prime, such as IN-100 and MAR M-200, generally are considered unworkable. On occasion, they can be cast as ingots, heat treated and successfully worked. However, the results tend to be unreproducible from billet to billet. This is due at least in part to macrosegregation, i.e., substantial variations in the local concentration of titanium or aluminum which result in local differences in the amount of eutectic gamma prime. This makes for a varied localized response to heat treatment. In addition, there often is residual microsegregation which can lead to incipient melting and cracking at relatively low temperatures. Cast single crystal alloys do not exhibit macrosegregation but they tend to be characterized by high volume fractions of gamma prime and some microsegregation.
Thermal mechanical working has been applied to gamma prime containing superalloys as exemplified by U.S. Pat. No. 3,147,155 to Lamb. His procedure for working a polycrystalline nickel chromium alloy involves solutioning the alloy to dissolve the gamma prime phase, cooling it to a temperature at which the gamma prime phase precipitates (but not the carbide phase) and then hot working it. Various patents of the present assignee are also informative. U.S. Pat. No. 3,676,225 to Owczarski et al describes how superalloys characterized by lower gamma prime volume fractions are strengthened by a procedure which includes solutioning, rapid cooling, cold working, aging, and warm working. In the procedure, applicable to alloys such as IN-718 and IN-901, the alloy is made workable by the rapid cooling step which avoids the formation of a precipitate. U.S. Pat. No. 3,975,219 to Allen et al discloses how high gamma prime content superalloys are deformed at constant temperature under critical conditions of strain rate, total strain, and temperature to produce a particular uniform dislocation density. A steep thermal gradient is then caused to pass through the workpiece to cause recrystallization of a progressive nature. U.S. Pat. No. 3,677,830 to Cox et al describes how precipitation hardening nickel base superalloys are heat treated, first at a temperature below the gamma prime solvus, and second, at a temperature equal to or just slightly below the gamma prime solvus. By these processes, there is obtained a uniform gamma prime precipitate pattern and controllable grain size, the grain size being inhibited by the formation of the gamma prime.
U.S. Pat. No. 3,642,543 to Owczarski and Oblak discloses a process for a thermal mechanical processing of alloys having gamma prime content in the range of 25%. The process involves solutioning followed by aging to create a finely spaced gamma prime structure with the spacing on the order of five microns or less; the material is then rolled at elevated temperature to deform it by at least 15%; thereafter the material is heat treated for stabilization of the substructure and additional precipitation hardening. It is indicated in the patent how single crystals of the alloys Udimet 700 and MAR M-200 were thermal mechanically worked to reductions of about 42%, with the result that yield strengths were increased by about 25%. Of course, rolling is a process characterized by high strain rates and variation in temperature of the workpiece during the time it is being worked. In the process of U.S. Pat. No. 3,642,543 a multiplicity of passes are used, each with 5-10% reduction and followed by re-heating of the workpiece.
The present day cast single crystal superalloys are in many ways simpler alloys than the older polycrystalline materials referred to in the paragraphs above. This is so because they are lacking the grain boundary strengtheners which are absolutely necessary for high temperature performance of polycrystalline materials. But this absence creates a limitation in that working of the material can readily produce recrystallization when the article is subsequently exposed to high temperature during processing or use. Once a high angle grain boundary forms in a single crystal, it becomes a point of substantial weakness. When used at an elevated temperature, a recrystallized single crystal material will tend to fail at the recrystallized grain boundary since there are no ingredients present in the alloy to inhibit movement along the grain boundary. Thus, in the practical manufacture of single crystal nickel superalloys, thermal mechanical working has not been considered an option.
It appears that the rolling process of U.S. Pat. No. 3,642,543 would incur a great chance of recrystallization during rolling, owing to the stated variation in bulk temperature, and the likelihood of cooling at edges, etc. But as reference to the patent will show, the early single crystal alloys contained carbon, boron, and zirconium and thus still had within them the ingredients which aid in grain boundary strengthening and also act to inhibit growth of recrystallized grains.
Generally, single crystal alloys and parts have been designed to withstand the highest use temperatures; but, they have had adequate properties at the intermediate temperatures to which they are subjected. Nonetheless, there is a constant search for improvements in properties in superalloys, and for materials which have very high yield and fatigue strengths at intermediate temperatures of gas turbine engine use.